Append v3 in module path and update travis configuration to use go modules

[psampaz]

According to official docs the module path should be

github.com/gemnasium/logrus-graylog-hook/v3

and not

github.com/gemnasium/logrus-graylog-hook

This is the official documentation for reference:

https://github.com/golang/go/wiki/Modules#semantic-import-versioning

• If the module is version v2 or higher, the major version of the module must be included as a /vN at the end of the module paths used in go.mod files (e.g., module github.com/my/mod/v2, require github.com/my/mod/v2 v2.0.0) and in the package import path (e.g., import "github.com/my/mod/v2/mypkg").
• If the module is version v0 or v1, do not include the major version in either the module path or the import path.

Also

https://github.com/golang/go/wiki/Modules#releasing-modules-v2-or-higher

Major branch: Update the go.mod file to include a /v3 at the end of the module path in the module directive (e.g., module github.com/my/module/v3).

[gravis]

You're absolutely right, thanks. I'll merge when it's not WIP anymore.